In a milestone for clean water, the new Brightwater treatment plant has begun work after more than a decade in the making and nearly $2 billion invested in the project.

The plant began treating sewage and discharged some of the first treated effluent into Puget Sound at the beginning of the month. The plant is so effective it is producing water 30 times cleaner than required under its state permit, and clean enough to use as reclaimed water.

“It’s a great benefit to Puget Sound,” said Mark Henley at the state Department of Ecology, which regulates pollution discharges into Puget Sound.

The plant is the largest wastewater project undertaken in King County in 50 years. Treated effluent from the plant flows through 13 miles of pipeline to Point Wells and is discharged 600 feet below the surface of Puget Sound, a mile off shore.

Work on planning the project began in the 1990s, and it has been under construction since 2006. Some work remains to be completed on odor-control facilities, fixing up the construction site at Point Wells and other projects. But overall, the project is about 99 percent complete, said Gunars Sreibers, project manager for King County’s Waste Water Treatment Division.

“It’s gratifying to see the plant performing well,” Sreibers said. “It’s a legacy for this region and will protect Puget Sound and provide capacity for growth for a long time to come.”

The new Brightwater plant serves South Snohomish County and North King County. Its construction was ambitious and complex, and it included construction of the treatment plant, a pump station and four deep-bore tunnels.